Puncturevine (Tribulus terrestris) is an annual plant that germinates between March and October. It grows quickly from seedlings to form sprawling mat-like plants that can become 10 feet in diameter! A large plant produces hundreds of small yellow flowers that mature into fruits with 2-4 short, painfully-sharp spines. It continues to germinate all summer, especially if there are small amounts of rain or landscape irrigation runoff. The plants are often found along the edges of roadways, pathways and driveways and are also common in disturbed areas around schoolyards, parking areas, construction sites and agricultural lands. The plant grows best in disturbed, sunny, bare areas and it enjoys most soil types.
People are the primary dispersal agents for puncturevine and after hitching a ride, people inadvertently disperse seeds into new areas. A vehicle (bike, car, truck or tractor) driving through an infested area can easily pick up hundreds of spiny fruits on tires and spread them far and wide. You can spread them on the soles of your shoes as well. The spiny puncturevine fruit causes painful injuries to pets and people alike. It's also the leading cause of bicycle flat tires on local trails and roadways.
If you learn to identify puncturevine when it first germinates, it's much easier to eliminate than when plants are large and seeds mature. For small infestations, use a hoe to cut the plant off at its taproot. For large plants, cut the plant off at soil surface with a hoe or pruners and bag it. It's not necessary to remove the entire root, but all of the aboveground crown (stem base) must be removed to prevent re-sprouting. Mowers and weed-eaters spread mature seeds and don't completely remove the crown, so they shouldn't be used. Loose fruits can be swept with a broom or sucked up with a yard vacuum and along with pulled plants, placed into a large plastic bag for disposal.
Infested areas should be monitored and the weeds removed before seed-set (late spring and early summer in this area). This will greatly reduce the number of plants the following year. Follow-up monitoring and removal may be needed, since puncturevine seeds can remain viable for 3-7 years.
Manual removal is the best control method for residential-scale properties, but large infestations require a different approach. There are a few pre-emergent herbicides that can be used, but they're only partially effective in preventing seed germination. Post-emergent herbicides can be used, but once the plant has set seed, killing the foliage does not reduce the number of spiny fruits so you'll still need to remove and bag up the plant. There are also two biocontrol weevils (see link below).
Puncturevine was first recorded in California in 1902 and used to be one of the state's most problematic weeds. In 1961, 2 weevils were introduced from Italy as biocontrol agents, successfully reducing the puncturevine population to manageable levels. These are a stem weevil (Microlarinus lypriformis) and a seed weevil (M. lareynii). They work well in low-traffic areas that have enough puncturevine to keep the weevils alive. These weevils can be purchased from biological supply companies but it's better to use locally-collected weevils. The Butte County Agricultural Commissioner's Office used to provide these weevils but funding cuts at the California Dept. of Food & Agriculture has eliminated this program. Now, if you want local weevils, you must collect them yourself.
The information below was provided by Rob Hill at the Butte County Agricultural Commissioner's Office on April 14, 2011:
A combination of both stem and seed weevils can be collected during the hot months, from the end of June through September. You will need a flat-bladed shovel and 1 or more 50-gallon clear plastic bags. The best collection areas are north of Chico along the secondary roads to the west of Hwy 99 starting at Anita Road (Cana Hwy, Broyles Rd., etc). Look for agricultural roadway turnouts that have puncturevine debris. Use the shovel to break the puncturevine root and scoop the entire plant(s) into the plastic bag. The weevils should be visible through the plastic.
Note: In a preliminary search for puncturevine in April, new plants were most prevalent along the edges of orchards. More specific weevil location information will be provided here in early July. When you park to collect the weevils, be sure that you're not parking in a puncturevine infestation; otherwise, you'll be responsible for spreading the weed too!